The present invention relates generally to radar antennas, and more specifically the invention pertains to a process for measuring the antenna pattern of a phased array antenna.
Traditional antenna measurements use 2D.sup.2 /.lambda. as a criteria for measuring an antenna under test (AUT) in the far field, where R is the distance from the AUT to a point in space, D is the aperture size of the AUT, and .lambda. is the operating wavelength. Problems arise when D becomes large, which increases the distance R, therefore more real estate is needed to measure the far field antenna pattern.
The requirements of the modern radars have led to the development of phased-array antennas that continue to grow in size, with current antenna lengths of at least 30 wavelengths. This can present a major problem in taking antenna pattern measurements, because of the requirement to measure these patterns in the antenna's far field. Consider a C-band antenna that measures 10 feet by 100 feet. Using the standard far field criteria of ##EQU1## where D is the aperture size, and .lambda. is the operating wavelength, the far field measurements would have to be taken at a distance of 23 miles from the antenna.
Fortunately, techniques have been developed to measure the far field pattern of the antenna under test on smaller indoor ranges. Advantages of using these ranges include: reduction of outside interference, reduction of testing time lost to poor weather conditions, the ability to do classified testing, and the reduction of electromagnetic transmissions into the environment. This last point is especially important with the current interest in environmental impact analysis. Unfortunately, there are still problems with these techniques. For example, the transformation of data collected by near field probing into the far field requires many sample points. Another problem is that a large reflector with a high surface tolerance restricts the use of a compact range.
Development over the years has led to techniques to measure the far field pattern of the AUT on smaller ranges. Using a small range reduces outside interference, eliminates testing time due to poor weather conditions, and provides the option of doing classified testing. Many of the previous smaller range techniques eliminate or lessen these problems. There are however, other problems with these techniques. For example, transformation from near field probing to the far field requires many sample points, the large size and high surface tolerance requirements of a reflector restricts the use of a compact range.
The task of reducing the range requirements while measuring the far field antenna patterns of large antenna arrays is alleviated, to some extent, by the systems disclosed in the following U.S. Pat. Nos., the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 5,204,685 issued to Franchi et al; PA1 U.S. Pat. No. 5,001,494 issued to Dorman et al; PA1 U.S. Pat. No. 4,998,112 issued to Franchi et al; PA1 U.S. Pat. No. 4,811,023 issued to Gelernter et al; and PA1 U.S. Pat. No. 4,704,614 issued to Poirer et al.
The patents of Franchi et al. disclose a method for measuring the far field antenna on a conventional far field range by applying a correction factor to the antenna. The Poirer et al. patent discloses measuring the near-field radiation by attaching a field sensing probe to a pendulum bob and mounting the antenna under a Foucalt pendulum. The entire antenna aperture can be scanned without moving the antenna. The motion of the probe covers part of an external sphere centered at the pivot point of the pendulum and having a radius equal to the length of the pendulum. Appropriate transformation of the measured near-field data gives the far-field radiation pattern. The remaining patents are of similar interest.
While the above-cited references are instructive, the need remains to measure the far field patterns of phased array antennas in a manner that reduces the range requirements. The present invention is intended to satisfy that need.